The present invention relates to improvements in strobo system used for cameras and, more particularly to a strobo system which has a circuit for exciting a transformer for charging a main capacitor in a non-self-excited fly-back system.
Recently, various electronic still cameras of internal strobo type, which use a strobo charging circuit of non-self-excited fly-back type, have become popular owing to their readiness for current control.
In the strobo charging circuit of the non-self-excited fly-back type, a charging voltage of a main capacitor is generated by an oscillating transformer which generates sharp changes of the coil current in response to the application of a predetermined pulse voltage. In an initial stage of charging, however, the oscillating transformer is more or less subject to the phenomenon of saturation and resultant generation of inrush current and increase of current consumption. This phenomenon deteriorates the charging efficiency and, depending on the setting of the charging pulse signal, leads to the operation failure of the system having the charging circuit due to the inrush current.
In such a strobo charging circuit of the non-self-excited fly-back type, referring to FIG. 8, when the "on" period pulse duration of a pulse voltage (labeled strobo pulse signal in the Figure) applied to the oscillating transformer in the strobo charging circuit is substantially equal, with as low terminal voltage across the main capacitor as zero to several ten volts, i.e., in an initial stage of charging, to the pulse duration t0 in the normal charging state, the following phenomenon arises.
When the "on" period pulse duration of the strobo pulse signal in an initial stage of charging is substantially equal to that in the normal charging state, the oscillating transformer may be magnetically saturated, causing an over-current (i.e., inrush current) (designated at A in the Figure) to flow through the primary side. This phenomenon arises since the pulse duration t0 is too long in correspondence to the charging voltage across the main capacitor. In this case, the secondary side current (designated at B in the Figure) is not used up on one cycle, resulting in energy accumulation in the transformer. The inrush current flows so long as this energy accumulation is continued.
The inrush current increases the current consumption, leading to reduction of the supply voltage and failure of operation of a control circuit of the system, which is furnished with power from the same power supply as for the strobo charging circuit.
As means for overcoming this drawback, it is well known in the art to suppress the magnetic saturation as a cause of the over-current by limiting the "on" period pulse duration of the pulse voltage to a relatively short period t, as shown in FIG. 9. However, continuing the oscillation of the oscillating transformer with the reduced "on" period pulse duration, gives rise to a drawback that the charging time is extremely increased, although the inrush current can be reliably suppressed.
In the meantime, to cope with a recent trend for reducing camera size, in many camera systems with such a strobo charging circuit the power supply for charging the main capacitor is also used for driving various actuators in the camera system and for supplying power to various control circuit of the camera.
Recently, however, a large variety of actuators are mounted in the camera, leading to a demand for high current supply capacity power supplies. On the other hand, size reduction of the power supply itself is another demand. Therefore, it is difficult to mount a power supply, which can withstand over-load.
When driving an actuator or the like, the common power supply bears a high load, and the power supply voltage is tentatively reduced.
In the meantime, it is desired to charge the main capacitor of the strobo charging circuit quickly and accurately. For this reason, the main capacitor is charged concurrently when driving various actuators or the like in the camera.
If any actuator is driven without concurrent charging of the main capacitor, the extent of the power supply voltage reduction not such as to give rise to any problem. However, when the actuator is driven while concurrently charging the main capacitor, the load on the power supply may exceed a rating, giving rise to various problems such as operation failure of the system.